1. Field of the Invention
The present invention relates to an optical waveguide.
2. Description of the Related Art
In the background of increase in information processing capacity and enhancement of information processing speed, information delay and heat generation problem in electronic interconnections have occurred, and solutions for these problems are being demanded. Such solutions include an on-chip optical interconnection technology and use of light in information processing itself. One of the technologies of using light in information processing is the silicon photonics, e.g., high-speed silicon optical modulators, promoted by Intel. Demand for the on-chip optical interconnection technology is indicated in the technology roadmap for semiconductors.
Optical interconnections and optical waveguides are also demanded to be smaller in size, but there is generally a limit in fine processing of optical functional elements due to diffraction limit of light. An effective measure is the “near-field” technology. Research has been intensively made about an optical switch and a plasmon waveguide using near-field light. A fine optical interconnection such as an on-chip optical interconnection is expected to be realized by the near-field related technology. However, since a light-emitting device and external optical information processing components function with propagating light, not with near-field light, conversion between propagating light and near-field light is indispensable in the entire system. However, as is known from the dispersion relation between light and plasmon polariton, the dispersion relation does not coincide between the propagating light and near-field light. In other words, conservation of energy (or frequency) and conservation of momentum (or wave number) are not established at the same time, which is a problem of so-called phase mismatching. Accordingly, the conversion efficiency between the propagating light and near-field light is poor. For example, in a fiber probe of a near-field optical microscope, the wave number vector is broadened at the tip end and hence has many wave numbers, with which the energy is matched only at a specific wave number, thus deteriorating the conversion efficiency.
In an optical device for guiding surface plasmon polariton which is one of the near-field light, coupling efficiency with an optical device for guiding the propagating light is also an important problem. As shown in FIG. 1, it is generally known that the dispersion curves are different between propagating light and surface plasmon polariton on a metal thin film. Therefore, it is not easy to establish efficient conversion between the propagating light and surface plasmon polariton.
At the level of research or experiment, there are known, as efficient coupling methods of propagating light and surface plasmon polariton, ATR (attenuated total reflection) methods using a prism in an Otto configuration or Kretschmann configuration. These methods are designed to adjust the wave number depending on the incident angle, and the coupler on the basis of such principle has a three-dimensional structure. Another example is an apparatus which receives the propagating light in the surface plasmon waveguide through a three-dimensional optical system, such as a beam splitter and a reflector (see, for example, JP-A 2004-20381 (KOKAI)) or a lens (JP-A No. 2006-171479 (KOKAI)). However, an information processing system and an optical circuit have a two-dimensional structure, and if such a three-dimensional coupler is used, a planar waveguide system cannot be constructed, which is convenient for integration. Hence, there is a demand for a two-dimensional coupler which controls the wave number of propagating light and can be provided between the propagating light waveguide and the surface plasmon waveguide.